1. Field of the Invention
The present invention relates to an electronic device formed by fabricating a semiconductor element (element utilizing a semiconductor thin film) on a substrate and an electronic apparatus utilizing such an electronic device as a display portion. Particularly, the present invention is a technique which is advantageously embodied in an active matrix EL (electroluminescence) display which is an electronic device.
2. Description of the Related Art
Recently, great advances have been made in techniques for forming TFTs on a substrate, and application of the same to the development of active matrix displays is in progress. Especially, TFTs utilizing polysilicon films are capable of operations at a high speed because they have higher field effect mobility compared to conventional TFTs utilizing amorphous silicon films. This has made it possible to control pixels with a driving circuit formed on the same substrate (insulator) on which the pixels are formed unlike the prior art in which pixels have been controlled by a driving circuit outside the substrate.
Such active matrix displays are presently attracting attentions for various advantages including compactness of the displays, improved yield and reduced throughput attributable to the fact that various circuits and elements are fabricated on the same substrate.
Various circuits and element portions having various functions are formed on the substrate of an active matrix display. Therefore, when elements are formed of TFTs, the TFTs are required to have different performance depending on the respective circuits and elements. For example, TFTs operating at a high speed are required for shift registers for generating a timing signal and the like, and TFTs having a sufficiently low off-current (a drain current that flows when a TFT is off) are required for switching elements for accumulating electrical charges.
In such a case, it is difficult to maintain performance requirements of all circuits or elements only with TFTs having the same structure, which can be a serious obstacle to efforts toward improved performance of active matrix displays.
It is an object of the invention to provide an active matrix type electronic device having a pixel portion and driving circuit portions provided on the same insulator, in which TFTs having appropriate structures are used depending on performance required for circuits or elements formed by the TFTs to provide high operating performance and reliability.
It is another object of the invention to improve the quality of images on an electronic device (particularly, an active matrix type EL display), thereby improving the quality of an electronic apparatus utilizing the same as a display portion.
In order to achieve the above-described objects, a principle of the present invention is that TFTs having optimum structures are allocated to each pixel of an EL display taking in view of the elements included in the pixel. That is, TFTs having different structures are present in the same pixel.
Specifically, TFT structures oriented toward lower off-currents rather than higher operating speeds are preferable for elements for which a sufficiently low off-current is the most important requirement (switching elements and the like). For elements through which a high current must flow as the top priority, it is preferable to use TFT structures oriented toward flow of high currents and suppression of very much problematic deterioration attributable to injection of hot carriers rather than the reduction of the off-current.
The present invention makes it possible to improve operating performance and reliability of an EL display by using different TFTs appropriately on the same insulator. The principle of the present invention is characterized in that TFT structures are optimized not only in a pixel portion but also in driving circuit portions for driving the pixel portion.